Apparatus for the purification of waste gas

ABSTRACT

Waste gas, in particular from plants for the chemical vapor-phase deposition and for the separation and etching by plasma processes, is purified from toxic agents by a method in which partial processes of the purification, such as thermal decomposition and oxidation, sorption of solid and gaseous reaction products, hydrolysis and cooling, are combined to act in a single reaction chamber. The reaction products of the waste gas burnt in a flame are led, directly in the combustion chamber, through a space filled with a finely dispersed liquid. This liquid forms a liquid film on all parts and inside walls of the reaction chamber. In a combustion chamber with a burner, the mentioned liquid distribution is obtained by a spraying device. 
     Effective purification from toxic agents, and little corrosion of the components of the reaction chamber are achieved, any growth of solid reaction products in the combustion chamber being avoided. The generation of secondary toxic agents during the combustion of the waste gas is avoided.

This application is a 371 of PCT/EP94/01985 filed 17 Jun. 1994.

This application is a 371 of PCT/EP94/01985 filed 17 Jun. 1994.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The invention relates to a method and an apparatus for the purificationof waste gas that develops in particular in plants for chemicalvapor-phase deposition (CVD) and for the removal or separation by plasmaprocesses.

2. Prior Art

Apart from substances generally regarded as unobjectionable, such asnitrogen and argon, waste gas of this kind contains toxic agents whichare completely removed or reduced to admissible limits by thepurification method. For the purpose of purification, the waste gas isconducted through the purification apparatus directly after leaving theproduction plant or after being collected from several plants.

Quite a number of methods are known for the purification of waste gas.It is very often effected by the sorption of the toxic agents forinstance in an oxidizing aequous solution. To this end, intense contactof the waste gas and the sorption agent takes place in a column, a spraytower or in any other kind of a washer. Toxic agents that are not, ornot efficiently, sorbed in this way can be converted by methods ofthermal decomposition by heating or by combustion and removed from thewaste gas. If the toxic agents themselves are not combustible, they areburnt in a flame by the addition of excess oxygen. The combustion takesplace in a burner-type combustion chamber, on the walls of which solidcombustion products such as SiO₂ will deposit. Since the separation ofsolid reaction products in the combustion chamber is not complete, and,in particular, since the combustion is mostly accompanied by secondary,gaseous toxic agents forming, multistage purification methods are knownto be used, in which partial processes of purification take place oneafter the other, such as thermal decomposition or oxidation, cooling,hydrolysis, sorption and ablution of solid reaction products (EP 0 347754 A1). Subsequent to the combustion chamber, the waste gas istransferred to a separate reaction chamber, there being subjected tointense contact with a sorbent.

To this end, the waste gas is conducted successively through acombustion chamber and at least one device that works on the washingprinciple. Also, apparatuses have been proposed for the purification ofwaste gas. To this end, the waste gas is conducted successively througha device comprising a combustion chamber and at least another devicethat works on the washing principle, the two forming a unitconstructionally (EP 0 347 753 A1). The combustion chamber and anexternal separate reaction chamber are separated by an inner wall.

A disadvantage of the waste gas purification based on the thermaldecomposition or oxidation of the toxic agents in a combustion chamberwith the formation of solid reaction products resides in that the latterwill deposit on the walls of the combustion chamber. With highthroughputs of toxic agents, thick layers will grow on these walls.After a while these deposits can become so thick that the process willbe strongly affected by faulty flow conditions or even become impossibleby "overgrowth".

Another disadvantage resides in the corrosion of the material of thewalls and of other components of the combustion chamber under theinfluence of gaseous reaction products of the combustion. This corrosionis increased critically owing to the high temperature of the waste gasburnt or to their composition, if the waste gas contains for instancehydrogen halogenide (e.g. HCL HF) and water vapor.

Furthermore, negative effects may reside in that secondary toxic agents,for instance dioxins, form upon the cooling of the burnt waste gas onthe way from the combustion chamber into a subsequent sorption chamberor sorption device.

Use is also made of absorption or adsorption methods in which a partialremoval of toxic agents from the waste gas is performed (Japanese patent812 2025, Japanese patent 6203 0525, Japanese patent 6213 6230).

However, the main disadvantage of these methods resides in that thetoxins are not eliminated, but that solid or liquid absorbents of thesame or even increased toxic contamination will originate and have to belodged in a dump.

OBJECT AND SUMMARY OF THE INVENTION

It is the object of the invention to improve the efficiency in thepurification of waste gas, in particular from CVD and plasma processes,with the aid of methods and apparatuses in which further partialprocesses such as sorption will take place in addition to the thermaldecomposition or oxidation of the toxic agents. Further, it is to beensured that the corrosion of parts of the combustion chamber is reducedwith a view to a prolonged service life. Yet another partial objectresides in dearly prolonging the operating time of the apparatus betweentwo cleaning cycles for the removal of deposits of solid reactionproducts. Moreover, it is to be ensured that no secondary toxic agentswill originate during the purification.

The method proceeds from that combustible gas for the production of aflame is supplied into a combustion chamber, and that the waste gas isled into this flame, the thermal decomposition of the toxic agents ofthe waste gas being effected in the flame. If toxic agents are to beoxidized, additional oxygen is added to the combustible gas or wastegas. The burnt waste gas contains solid and/or gaseous reaction productsof the toxic agents.

According to the invention, a liquid is fed directly into the combustionchamber, where it is sprayed in such a way that it is finely dispersed,filling the space above the burner and above the combustible gas flame.In addition to the thermal partial processes, this helps in effectivelyperforming a number of further partial processes for waste gaspurification directly in the combustion chamber. They consist in thatthe flow of burnt waste gas is conducted through the space of finelydispersed liquid and then discharged from the combustion chamber. Ifwater is used as a liquid, solid reaction products such as SiO₂ can bewashed out. Water is also suitable for the sorption of gaseous andvaporous reaction products, for instance NO₂, for the hydrolysis forinstance of HCl or HF, and also for the cooling of the flow of burntwaste gas. Components that are reactive, in particular neutralizing, canbe added to the water. In this way, acids are prevented from formingright in the combustion chamber.

The direct contact of the liquid with the flow of burnt waste gas withinthe combustion chamber helps increase the efficiency of the waste gaspurification decisively. The formation of solid reaction products takesplace virtually entirely in the combustion chamber. Thus, any sorptiondevices that may be additionnally connected are not contaminated andtheir operation is not affected.

The direct contact of the hot flow of burnt waste gas with the coldliquid helps prevent the formation of secondary gaseous toxic agents noteasily soluble in water, such as dioxins. The reason for this probablyresides in that the hot flow of waste gas is cooled suddenly by a hightemperature gradient.

The liquid sprayed in the combustion chamber deposits as a film whencontacting the walls of the combustion chamber. This liquid filmenlarges the effective interface towards the liquid which the flow ofgas contacts. The liquid film on the walls thus contributes to thesorption of solid and gaseous reaction products from the burnt wastegas, to the reaction with secondary toxic agents and to the cooling. Inparticular, this film of water prevents the deposition of solid reactionproducts of the combustion on the walls. The growing of thick layersthat impede the process is avoided.

This liquid film also helps avoid the direct contact of the hot burntwaste gas with the walls. Corrosive constituents will become effectiveon the walls only after being cooled and sorbed in the liquid, and thusin a lower concentration. As a result, the corrosion is strictly reducedand the service life of the components of the combustion chamber isprolonged.

The liquid film flows downwards on the walls of the combustion chamber.The liquid that is discharged in the vicinity of the bottom of thecombustion chamber takes along the sorbed solid and gaseous constituentsof the combustion and reaction products of the burnt waste gas.

The periods for which the combustion chamber can work before anothercleaning will become necessary are multiplied.

The method is put into practice with the aid of an apparatussubstantially comprising a perferably rotationally symmetricalcombustion chamber with a burner, and inlets for the combustible gas andthe waste gas that contains toxic agents. As the case may be, additionaloxygen is added to the combustible gas or the waste gas. The combustiblegas used may for instance be a hydrogen-oxygen mix or a naturalgas-oxygen mix. The combustible gas flame forms above the burner. Theheated or burnt combustible and waste gas flows through the combustionchamber in the direction towards a connection for a ventilation systemfor the discharge of the purified waste gas. In accordance with theinvention, the apparatus comprises a spraying device for a liquiddisposed in upper portion of the combustion chamber. This sprayingdevice consists of a hollow-cone nozzle and a solid-cone nozzle, whichare both fixed in the axis of the burner. The distance between theburner and the hollow-cone nozzle is set such that it is sufficient forthe formation of the combustible gas flame and for the complete thermaldecomposition and oxidation of the toxic agents. The hollow-cone nozzlesprays the said liquid in such a way that an angle sector remains freefrom liquid around the axis, the burner being disposed in this anglesector. This angular spread is preferably set to an angle 2α exceeding50°. The fact that the liquid is sprayed by the hollow-cone nozzleensures that the combustion flame can form in the space between theburner and the hollow-cone nozzle without being extinguished by theliquid; on the other hand, the hollow cone formed by the liquid preventsthe flow of burnt combustible and waste gas from contacting parts of thecombustion chamber, for instance the nozzle and liquid inlets, withoutprior contact with the liquid. The axial distance between thehollow-cone nozzle and the solid-cone nozzle exceeds the radius of thecombustion chamber. By means of the solid-cone nozzle, the liquid issprayed in the form of a solid cone of an angle of 2β exceeding 90°.Angles of more than 90° are appropriate for filling the space, betweenthe nozzles, of the combustion chamber as completely as possible withfinely dispersed liquid.

Owing to the high densitiy of liquid droplets, the hollow cone of finelydispersed liquid acts as a "shield" for the liquid sprayed by thesolid-cone nozzle, preventing any inadvertent penetration of liquid intothe combustion chamber.

The rear of the hollow-cone nozzle is provided with a face convex in thedirection of the flow of gas, the diameter of which exceeds or equalsthe diameter of the burner. This helps ensure that the liquid of thesolid-cone nozzle is not sprayed directly into the burner. The convexface works as a splash guard. Any liquid dropping off it will be out ofthe reach of the burner. Consequently, the formation of the burningflame and the combustion of the toxic agents is not affected by liquidfrom the solid-cone nozzle.

The flow of burnt combustible gas and of toxic agents is conductedthrough the space that is filled with finely dispersed liquid. This iswhere the said non-thermal partial processes of waste gas purificationtake place. Suitably, an additional sorption device, such as adehumidifier, a washer or a filter can top the combustion chamber via asuction conduit. This contributes to further reducing the toxicconstituents of the waste gas purified in the combustion chamber. Thetopping of the combustion chamber by an additional sorption device isespecially advantageous if the device acts selectively on toxic agentsthat have not yet been removed from the waste gas in the combustionchamber.

The consumption of sorbents can be strictly reduced if the sorbentdischarged from the combustion chamber and containing reaction productsof the waste gas combustion and of the reaction of the burnt waste gaswith the water, is recovered in a sorbent regenerative system, and ifthe sorbent is circulated.

Another suitable embodiment of the apparatus according to the inventionresides in inserting a layer of fillers in part of the space between thetwo nozzles. Thus the contact face of the liquid and the flow of gas isincreased in a manner known per se, the fact that the flow of burnt gasfirst contacts the finely dispersed liquid not being negativelyaffected. The sprayed liquid from the hollow-cone nozzle as well as fromthe solid-cone nozzle deposits on the walls of the combustion chamber.It forms a liquid film on these walls, flowing downwards to the bottomof the combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the invention will become apparent from thedescription of a preferred embodiment of the apparatus, taken inconjunction with the drawing. FIG. 1 illustrates a longitudinal sectionof the apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 illustrates an alternative embodiment of the invention accordingto FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE INVENTION

The apparatus according to the invention substantially comprises acylindrical combustion chamber 1 of stainless sheet steel. It has adiameter of 18 cm and a height of 60 cm. A burner 3 is disposed in thevicinity of the conical bottom 2 of the combustion chamber 1,combustible gas H₂ and O₂ being supplied to the burner 3 via an inlet 4.The burner 3 has a diameter of 25 mm.

The combustible gas flame 6 forms above the apertures 5. The waste gaswith the toxic agents is supplied to the burner 3 via the inlet 7. Itenters the combustible gas flame 6 centrally through the drill hole 8.The thermal decomposition and oxidation of the toxic agents takes placein the combustion chamber 9. The arrows 10 mark the direction of theflow of burnt combustible and waste gas. At a distance of about 40 cmand 55 cm, respectively, from the bottom 2, a hollow-cone nozzle 11 anda solid-cone nozzle 12 are disposed axially one after the other, whichare fastened to the supply lines 13 and 14, respectively, for water usedas a sorption agent. The hollow-cone nozzle 11 sprays water in such away that the angle sector 15 (2α≈80°) is substantially free fromdroplets and that the sprayed water fills a space 16 that isapproximately in the form of a hollow cone. Finely dispersed water fromthe solid cone nozzle 13 virtually fills the entire space 17 (2β≈180°)between the two nozzles. The sprayed water from the spaces 16 and 17meets with the inside walls of the combustion chamber 3, forming aliquid film 18. It flows downwards on the walls into the conical bottom2, where it is discharged from the combustion chamber 1 via an outlet19.

As shown in FIG. 2, another suitable embodiment of the apparatusaccording to the invention resides in inserting a layer of fillers 22 inpart of the space between the two nozzles 11, 12 on a supporting grid23. This results in an increase of the contact face of the liquid andthe flow of gas without negatively affecting the flow of gas through thefinely dispersed liquid.

The consumption of sorbents can be strictly reduced if the sorbentdischarged from the combustion chamber and containing reaction productsof the waste gas combustion and of the reaction of the burnt waste gaswith the water, is recovered in a sorbent regenerative system 24, and ifthe sorbent is circulated.

The rear of the hollow-cone nozzle 11 is in the form of a spherical cap20 of a radius of 40 mm, the diameter of the cap being 60 mm.

The purified waste gas is discharged via the connection 21 into theventilation system.

The purification apparatus is completed by ignition devices for thecombustible gas flame and by checking devices for the flow ofcombustible gas and the water as well as by sensors and actuators forincreasing the process safety.

Example for the performance of the process:

In a CVD plant for the deposition of phosphorus gas, waste gasquantities of in each case 55 l/min develop during the wafer separationas well as during the in-situ cleanings, performed alternately, of theprocess chamber. The waste gas developing during the deposition containsSiH₄, PH₃ as toxic agents and oxidation products of these agents. Thewaste gas developing during the in-situ cleaning of the process chambercontains C₂ F₆ as a toxic agent, which is thermally decomposable andthen hydrolyzable. In either process, nitrogen is the substantialconstituent of the waste gas, preferably ranging from 90-95%.

The waste gas is supplied to the purification apparatus via a waste gasconduit. In the combustion chamber 1 of the purification apparatus, aflame is kept burning by the supply of 15 l/min of hydrogen and 10 l/minof oxygen to a burner 3. After the complete combustion of the hydrogen,there is still heated excess oxygen in the burning flame. The waste gasis supplied in the axis of the burner 3. It is heated. The toxic agentsSiH₄ and PH₃ are oxidized to silicon dioxide and phosphorus oxide aswell as well as water. C₂ F₆ is decomposed to HF and CO₂.

The upper third of the combustion chamber 1 is filled with droplets ofwater. Approximately 10 l/min of water are additionally fed into thecombustion chamber and sprayed. When the droplets of water meet with thewalls of the combustion chamber 1, a film of water 18 forms on thelatter. The water flows downwards on the walls. It is collected in thevicinity of the bottom 2 of the combustion chamber 1 and discharged. Theburnt combustible and waste gas of a temperature of about 1000° C. flowsvertically from the burner 3 through the space 16 filled with waterdroplets. The solid silicon dioxide is colloidally solved in water, thephosphorus oxide reacts with water to phosphorus acid, the HF tohydrofluoric acid. The purified waste gas is discharged along thesurface of the cover of the combustion chamber 1. When the hot burntcombustible and waste gas meets with the droplets, it is cooled down toless than 50° C. Reaction products are sorbed in the water droplets anddischarged together with them from the combustion chamber.

We claim:
 1. An apparatus for the purification of waste gas comprising acombustion chamber and burner having inlets for a combustible gas and awaste gas having toxic agents said inlets arranged at the bottom of thecombustion chamber and an outlet at the top of the combustion chamber,wherein an upper portion of the combustion chamber (1) is provided witha spraying means for a liquid, said spraying means being disposedcoaxially with a central axis of the burner (3) and of said combustionchamber, the spraying means consisting of a hollow-cone nozzle (11) andof a solid-cone nozzle (12), wherein said hollow-cone nozzle (11) spraysthe liquid in the form of a hollow spray cone, wherein said solid-conenozzle (12) is arranged above-said hollow-cone nozzle (11) along saidcentral axis, the hollow spray cone generated by the hollow cone nozzlehaving a liquid-free inner space having an angle of aperture (2α)exceeding 50°, and wherein the solid-cone nozzle (12) produces a sprayhaving an angular spread (2β) more than 90° wherein a rear of thehollow-cone nozzle (11) is provided with a convex surface facing towardsaid solid-cone nozzle (12), a diameter of said convex surface exceedingor equalling the diameter of the burner (3).
 2. An apparatus accordingto claim 1, wherein a layer of filler means for increasing a contactbetween the liquid and a flow of the waste gas is disposed on asupporting grid, said layer of filler means being inserted in part ofthe space (17) between the two nozzles.
 3. An apparatus according toclaim 1, wherein the cylindrical combustion chamber (1) is provided witha sorbent regenerative system via the outlet (19).